This section is omitted as there are no other related applications at present.
1. Field of the Invention
The field of this application is multimedia digital optical communications, including telephony, data and video, both switched and non-switched.
2. Background Art
A search was conducted at the Boston Public Library, a U.S. Patent Depository. A total of 25 listings are on file for patents issued under subject heading: Telephone, Class-LightPhone, subclassxe2x80x94Optical Communications (359/149). No issued patent covers the design and application contemplated by LightRamp 2000. All extant patents predated 1986.
Attempts to date to employ fiber optics in local access networks have focused on two architectures: (1) corporately fed passive fiber to the subscriber, using Tree and Branch architecture, featuring optical splitting of the main feed signal for distribution to separate destinations, and (2) fiber to the premise, in which individual fibers carry trunk signaling to an interface box, the signals then being converted to conventional telephone or cable drops. The first class of fiber to the residence employs a single main optical feed in which signals for multiple users are multiplexed. The corporately fed optical signal is then split into individual tributaries to each subscriber. In the British Telecom""s TPON corporately fed fiber to the home scheme, long wavelength laser driven optics carry time division multiplexed traffic to multiple homes via optical splitter and propagated over a tree and branch structure. At the residence/premise the arriving traffic is then demultiplexed from the incoming high speed stream. Coding in the form of scrambling is utilized to prevent one subscriber from receiving another""s messages.
A second type of Fiber to the Home (FTTH) employs a dedicated fiber pair to the premises, using digital trunk signaling. The fibers terminate at an interface unit at the entry to the premise. The remaining connections to telephones or other terminals are made over metallic wire, using conventional DC loop signaling. This type of design is exemplified by the experimental system tested by Bell South/Bell Northern Research at Heathrow, Fla., (Refs. 1 and 2). The Heathrow trial system successfully carried ISDN over a dedicated fiber pair to the premise, using trunk type digital signaling to the residence. The final drop to the telephone instrument from a side wall mounted demarcation interface box reused existing household wire pairs. These pairs carried out DC loop supervision and BORSHT functions generated by a SLIC located in the demarcation box. In effect, this arrangement was a single channel optical digital loop carrier to the residence.
The Heathrow scheme was characterized and limited by the following attributes:
(1) In using conventional tip and ring line supervision over the wire pair(s) extending from the demark unit to the telephone instrument, no new protocols were devised to achieve line supervision.
(2) fibers were employed for the Basic Rate ISDN(BRI) 2B+D service only, with one fiber in each direction. No other service was offered over the fiber pair.
(3) Laser technology was used at Heathrow, resulting in costs that are higher than permissible to satisfy the BellCo""s deployment criteria; i.e., that first costs for the fiber must be comparable to a conventional metallic pair in order to justify an installation decision. More economical ELED technology was not then available. This dedication of transmission capacity to specific service, rather than combining multiple services one single medium, characteristic of traditional telephone practice, is not compliant with ITU ISDN goals.
(4) no video or wide band data was carried on the fiber, nor was ATM traffic attempted or feasible. Video (NTSC) was carried over separate coaxial cable in channelized analog form.
The Heathrow experiment, although technically successful, did not, in the opinion of the lead investigators from Bell South and BNR, lead to a design that was ready for introduction to the market. This was due to the a priori constraints imposed by the testers, as well as by the state of the electro-optical art prevailing in the late 1980 time frame. A number of major shortcomings to the overall design, admitted by the researchers, included the basic flaw of substituting fiber for wire on a per service one for one basis. This assumption failed to take advantage of the economics to scale and scope possible with fiber, which allows additional services to be added at minimal marginal cost. No such economies were sought or realized at Heathrow.
Limitations of Hybrid Fiber-Wire Connections to Premise
Almost all previous and current attempts to convey multi-media services traffic (voice, data, video) via fiber to a premise all use fiber to an intermediate nodal concentrator located in the neighborhood. The individual services are then separated and each delivered over separate media to the user. Typically, voice is conveyed over a twisted metallic pair, video is conveyed over coaxial cable, and data over two wire pairs. In every instance field tested to date, telephone service used conventional two wire metallic loops to provide both line supervision signaling and signal conveyance.
The three most frequently mentioned hybrid fiber/metallic designs are Fiber to the Curb(FTTC), Hybrid Fiber/Coax (HFC) and Asymmetrical Digital Subscriber Link(ADSL). In FTTC a node is located in the neighborhood within 800 feet of the premise/residence. This distance, fixed by the signaling capability of the wire pair, means that a local node can reach 16 residences in rural areas and 32 residences in cities. Alternative HFC designs planned by cable MSOs and some LECs utilize fiber to the service area (FSA), with the coaxial distribution reaching out to about 500 homes. In HFC/FSA the required cable run exceeds 1400 feet, requiring line extension amplifiers. A single coaxial cable serves many subscribers with one way video service. In some cases two way data service is also offered. In that event two way amplifiers are needed beyond 1000 feet. ADSL has a different feeder structure, but is nearly identical to FTTC over the last xe2x80x98milexe2x80x99.
The use of a fiber feeder and metallic distribution/drop combination in FTTC and ADSL carries with it the following burdens which are eliminated in the all passive structure of Light Ramp 2000: (a) an active intermediate node is required to convert and distribute the signals and thirty two or more such nodes are required per square mile; (b) the active intermediate node, also called an Optical Network Unit (ONU), delivers dial tone, ringing, and on hook/off hook status, among other line supervisory functions in order to support telephone service. They are known as the BORSHT functions and are implemented by a subscriber line interface circuit (SLIC). The BORSHT functions require a current carrying DC metallic loop and are not portable over fiber, which is electrically non-conductive; (c) to perform its functions the ONU requires an electrical power source, typically provided via a separate power feed from the Telco Central Office or local remote terminal(RT); finally, (d) the two wire metallic drop is severely limited in bandwidth and distance, restricting true broadband services to one direction only over an 800 foot drop. In effect this capability includes only entertainment and highly asymmetrical data retrieval services. Thus, the introduction of true symmetrical two way wide band digital services, requiring greater than 1-2 Mbps in each direction, is severely restricted and is generally precluded over conventional telephone wire over distances exceeding 1000 feet.
In view of their wire limitations, Hybrid Fiber-Coax or Hybrid Fiber-Metallic systems are considered as interim configurations, in anticipation of eventual extension of fiber directly to the home(FTTH) or business premise(FTTB). The Regional Bell Operating Companies have stated that FTTH is the eventual goal. Knowledgeable practitioners have stated that the high peak data rates anticipated in the loop part of the network will push optical fiber xe2x80x98all the wayxe2x80x99 into residences. (Ref. 3)
Limitations In the Traditional Telephone D.C. Local Loop
For many decades telephone connections to the premise have used DC powered metallic loops extending from telephone company subscriber line interface circuits (SLICs), located either at the central office (switch) or, more recently, at local neighborhood concentrators. Service provided via these local concentrators is referred to as Digital Loop Carriage (DLC) where the connection back to the telco central office is made by a trunk carrying 24/48 digital channels.
This DC current loop is a typically a 24 or 26 AWG twisted wire pair extending up to 12 kft and terminating, when the hook switch is in the xe2x80x98off hookxe2x80x99 or xe2x80x98onxe2x80x99 position, into a 100-150 ohm loadxe2x80x94the telephone. The combination of battery, wire loop and telephone constitutes a DC conductive path over which dial tone and ringing and other audibles are provided and on/off hook line supervision is exercised. The traditional loop also carried address signaling, either dialing pulses from rotary phones, or more recently tone pairs from touch tone sets. These telephones are analog instruments, because the standing loop current modulated by the carbon microphone is nearly the exact analog to the undulations of the human voice. This is Alexander Graham Bell""s principle but not only invention.
Thus the continuing use to the DC loop for line supervision virtually guarantees that multiple wires continue to extend to the premise and that service over the single telephone line will be severely restricted. This practice defeats the goal, established by the International Telecommunications Union (ITU) for the Integrated services digital Network (ISDN), of integrating all forms of transmission over a single medium.
Limitations of four wire ISDN
In digital ISDN the DC loop merely functions to perform on/off hook line supervision and to power the phone. The standing loop current is no longer modulated by the human voice, or any other continuous tone. Instead, voice samples are quantified into a sequence of digital samples which become a fast digital representation of the original, to be reconstructed at the receiving end. In between the two telephones, the information flow is entirely digital and an additional metallic pair is required to achieve full duplex operation.
Since a voice signal in ISDN is fundamentally digital there is no analog representation of the voice signal anywhere in the channel. Voice digitization occurs within the telephone instrument and is performed by a circuit known as a coder/decoder (CODEC). Voice occurs only in its natural form at the output of this circuit, where the receiving CODEC and transducer change the discrete electrical signal back to acoustic. In becoming a purely digital instrument, the ISDN telephone inherently accommodates other forms of digital traffic, such as base band data from a computer, or other digitized signals, such as compressed video. However, there are physical limits that prevent metallic ISDN from providing a general purpose digital telephone service.
In summary the limitations of four wire ISDN are:
(a) two wire pairs are needed to support full duplex operation of ISDN
(b) not all wire pairs are eligible and testing is required to determine eligibility
(c) wire based ISDN is subject to distance limitations
As a result of these limitations ISDN, if provided over wire pairs, requires a local neighborhood node to serve more distant users; or else, only these within the acceptable signaling distance qualify for service. This constraint penalizes neighborhoods with older embedded wire plant.
Limitations of Alternative Conductive Wire Bases Schemes: Asymmetrical Digital Subscriber Link (ADSL/B-ADSL) and Fiber to the Curb (FTTC)
New or Revised Telephone Loop Designs are now being promoted for the purpose of expanding the bandwidth of Hybrid Fiber/Metallic designs to deliver a greater array of services, with strong emphasis on video for entertainment purposes. These can roughly be categorized as (1) Enhanced Wire Based Schemes, and (2) Coaxial delivery schemes, usually with a fiber optic feeder.
The enhanced wire based scheme, ADSL, is currently used in limited trunk applications. ADSL is being strongly promoted on the basis that it avoids presumably expensive installation of fiber and takes advantage of existing conductive wire plan. Asymmetrical service is justified on the basis that Internet access currently enjoys a 10:1 asymmetry in favor of downstream traffic and that traffic is dominated by entertainment video. This ratio changes dramatically when two wave digital services are introduced.
ADSL is severely limited in both signaling distance (less than 12 kft tested), and data carrying capacity. This limitation is particularly noticeable in the upstream direction. Furthermore, as data rate goes up, signaling distance goes down. The highest rate claimed, not yet proven operationally, is 25 Mbps with a corresponding reach of 3 kft. Only 1.544 Mbps over two pairs has been demonstrated up to 12,000 feet.( Ref. 4 and 5). ADSL therefore generally needs a local hub or roadside CO supplied by feeder trunks in order to reach most subscribers. The feeder portion of the ADSL/B-ADSL network does not yet exist, makes extensive use of fiber, and will be costly to install. Reuse of existing or planned SONET facilities is not an option.
ADSL is an inherently asymmetrical transmission facility which achieves high speed in one direction, typically 1.5 to 6.2 Mbps, and provides full duplex digital data at 64-640 Kbps. These rates are only achievable over 50 percent of telephone lines. Single line plain old telephone service (POTS) is supplied over separate wire pairs but over the final drop it occupies the 0-4 KHz band in combination with other signals on the single wire pair to the residence.
The current ADSL 6.2 Mbps capacity incorporated in ANSI T1E1.4 Standards, will be too slow to support one or two MPEG-2 video channels, let alone broadband services. At least 25-50 Mbps would be needed, and has yet to be demonstrated. The development at 25 Mbps, now at the laboratory stage, is said to be capable of reaching about 3 kft, serving 100 or more homes. A 52 Mbps prototype transceiver extending 700-800 feet was scheduled for beta testing in the second quarter of 1996. (References 4 and 5). This receiver, being developed for Fiber to the Curb(FTTC), will most likely also be adopted for ADSL. It is apparent from published data that the 400 feeders to the ONU nodes from the CO, required in both ADSL and FTTC for a model town, total 384 cable miles of costly OC12/24 six fiber asymmetrical transmission cable. These constitute a much greater combined investment in fiber and optoel-ectronics than the simple single ring cable of the Light ramp 2000 design. The latter reuses current and planned SONET fiber.
Limitations of ADSL and FTTC are seen more clearly when deployments are compared in a model town used by ADSL designers. This model is a 4xc3x974 mile square (16 sq. mi.) containing 10,000 homes, of which 6000 are in the outskirts and 4000 in-town. In-town is considered the inner 2xc3x972 square (4 mi.square).
Using signaling rate of 52 Mbps constrains the reach to each residence to 800 feet, theoretically serving 16 rural homes and 32 city homes from each node. In the model town, street mile density in the outskirts is approximately 70 homes/mile, allowing an 800 foot drop to reach 16 residences. Altogether 504 nodes would be required for the entire town. By contrast, with passive fiber links to the residence as in the preferred mode of this application, only 16 RTs ( one per square mile) would be needed.
The inventors and proponents of ADSL (Reference 6) admits that this scheme is an interim solution, awaiting the arrival of fiber to the premise.
Conclusions Regarding Limitations of Fiber to the Curb (FTTC)
Fiber To The Curb(FTTC) is a hybrid design in which fiber comes to the immediate neighborhood, serving from 8 to 32 homes. Drops to the premise will be conventional wire pairs carrying traffic at either 25 or 52 Mbps. Traffic from the Telco hub to the neighborhood concentrator, known as an ONU (optical network unit) is carried over both coaxial cable and fiber. In addition to requiring power to the ONU, FTTC handles switched video on a compressed basis and does not convey switched uncompressed broadcast video, as will Light Ramp. Selection and switching of the video content occurs at the head end, generally colocated with the telephone central office. Some distribution switching or routing of signals occurs at the ONU. This arrangement requires up to two OC12 (622 Mbps) feeders to each ONU.
FTTC incurs unattractive costs due to the need to provide a dedicated coax or reengineered wire pair to each residence rather than providing a more economical shared coax, as in conventional CATV and HFC. As we show later, Light Ramp provides a dedicated fiber, with sharing of the single 1fiber by many services. This is an economy of scope, which however is competitive with POTS over metallic pairs on both a first cost and operating cost basis. Therefore additional services and their associated revenue streams are not needed to justify, decision to install Light Ramp 2000.
Essential differences arc that FTTC requires a local concentrator (ONU), power to that concentrator, and provides many channels of compressed video. Over the feeder portion of the network telephone lines are also provided over DLC rather than on individual pairs, as in ADSL. Light Ramp is totally passive over the connection from Telco to the premise, requires no local node and therefore needs no power down the line or from local supply. Therefore the differences between Light Ramp 2000 and FTTC are fundamental and no simple upgrade of FTTC will transform it into Light Ramp.
The primary object of Light Ramp 2000 is to obviate the above noted disadvantages of the prior art.
It is a further object to provide secure, switchable dial up nearly error free wide band local access to the Public Switched Networks(PSNs) over totally passive fiber optic architecture, such that ITU criteria for ISDN and B-ISDN (Refs. 7 and 8) are satisfied.
It is a further object of the design to dispense with existing metallic loops and coaxial cables for the last mile by providing a new set of signaling protocols needed to support line supervision signaling in place of the DC metallic loop.
A further object is to provide dial up capability for both multimedia and conventional telephone calls, including ISDN, over fiber from anywhere within the 6-8 KM short or 10 KM long reach of the fiber extending from the local node.
A further object is to provide multimedia services over a single medium in a cost effective manner, such that local Exchange Carrier first cost installation criteria are met.
These and other advantages and objects are provided for herein. A diverse routing of a passive fiber pair to the premise is provided by the ring architecture of Light Ramp 2000. By virtue of its signaling distance and bandwidth, this design will be capable of carrying multiple digital signals at rates in excess of 100 Mbps both to and from business and residential premises over one or both fibers. SONET OC-3c (155 Mbps) or greater capacity is offered in both directions by using wavelength division multiplexing (WDM) on each branch of the ring. Thus simultaneous transmission of uncompressed video as well as multiple voice and data channels are supported. Broadband services such as distance learning, video conferencing, and telemedicine, as well as dial up digital voice and data transmission over the single medium are thereby enabled. Bandwidth and distance constraints of the metallic wire pair are eliminated, and an improvement factor in data throughput of over 1000 to 1 is achieved relative to switch 56 kbps modem links.
Inherent economy in the design achieved by use of dedicated passive fiber(s) over xe2x80x98the last milexe2x80x99 via a single shared cable, in combination with low power ELEDs, will allow service to the residential user at more affordable rates than possible with the use of multiple conductors. In addition to low first costs, less than that of a single telephone wire, operating costs are much lower. Once installed, the inert non-conductive glass medium will be virtually maintenance free and outside plant maintenance actions will be limited to occasional visits to a single on pole cross connect box.
Light Ramp introduces a new set of supervision signaling protocols in place of the DC current loop. This digital signaling scheme provides greatly expanded functionality suitable to broadband network, and all of the vertical and enhanced (CLASS) services are readily accommodated on all telephone channels. In addition, ATM broadband multimedia services can be accessed on a dial up basis from the local ATM switch, a feature that provides significant economies over the current dedicated connections and associated separate signaling channels.